Genetically encoded fluorescent sensors for detecting intracellular signalling through diacylglycerol pathways

ABSTRACT

Described herein are novel fluorescent sensors for Diacyl Glycerol (DAG) and phosphatidylinositol 4,5-bisphosphate (PIP2) that are based on circularly permuted fluorescent proteins. These sensors use less visible spectrum than FRET-based sensors, produce robust changes in fluorescence, and can be combined with one another, or with other sensors, in a multiplex assay on standard fluorescent plate readers or live cell imaging systems.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119(e) from U.S. Provisional Application Ser. No. 61/611,406, filed Mar. 15, 2012, the contents of which are incorporated herein in their entirety by this reference.

GOVERNMENT SUPPORT

This invention was made with government support under NIH grant 1R43MH096670-01A1 awarded by the National Institute of Mental Health. The government has certain rights in this invention.

REFERENCE TO SEQUENCE LISTING

This application contains a Sequence Listing submitted electronically as a text file by EFS-Web. The text file, named “6666-2-PCT_Sequence Listing_ST25”, has a size in bytes of 354 KB, and was recorded on Mar. 15, 2013. The information contained in the text file is incorporated herein by reference in its entirety pursuant to 37 CFR § 1.52(e)(5).

FIELD OF THE INVENTION

The field of the present invention is design and construction of fluorescent biological sensors for detection and measurement of intracellular analytes.

BACKGROUND OF THE INVENTION

Cell signaling involves the concerted activity of multiple second messenger pathways. It is the balance of these different signaling components, coordinated in both space and time, that ultimately dictate the response of the cell. While this is well understood in theory, the practice of measuring signaling is often reduced to two time points—before and after drug—and to a single second messenger. When kinetic measurements of signaling are possible, a new level of precision and insight guide new experiments and optimized assays. In the cases that it has been possible to image multiple components of a signaling pathway in the same cells (1-5), the interplay between the different components has provided new insights into the biological system and the downstream consequences of a drug's actions.

Multiplex sensors capable of simultaneously detecting different signaling components serve an important role in understanding complex biological pathways and assessing the biological relevance of a particular drug (8). For example, many drugs act at G-protein coupled receptors on the cell surface. Some of these receptors couple to the heterotrimeric protein, Gq, which activates phospholipase C (PLC). PLC in turn cleave PIP2 to produce two second messengers: diacylglycerol (DAG), which remains in the plasma membrane, and inositol triphosphate (IP3), which diffuses through the cytosol to release stores of intracellular calcium ions (Ca2+). This coordinated increase, in both DAG and cytosolic Ca2+, triggers the activation of conventional isoforms of protein kinase C (cPKC) which then phosphorylate many different targets. To unambiguously resolve PLC pathway activation, and to better understand the kinetics of these coordinated, parallel signaling processes and their significance in health and disease, multiplex sensor systems are needed that can simultaneously measure multiple molecules such as DAG, PIP2 and Ca2+.

Optimal multiplex sensors must satisfy a number of criteria. First, they must be capable of working in living cells and provide kinetic data for each signaling pathway. This means they need to work in living cells and provide strong signals that can be sampled at 10 Hz. Additionally, each sensor needs to consume as little of the visible spectrum as possible so that there is minimal crosstalk with other sensors. Furthermore, each sensor has to specifically detect the analyte at physiologically relevant concentrations.

Fluorescent protein-based sensors meet many of the design criteria: they work in living cells, they produce strong signals that can be sampled repeatedly and quickly, and the protein domains they carry have evolved to specifically detect a particular second messenger (1). However, early sensors based upon Forster Resonance Energy Transfer (FRET) between two different fluorescent proteins, rarely produce the sort of robust signals necessary for automated detection. Furthermore, the broad absorption bands of the donor and acceptor fluorophores consume most of the visible spectrum (12, 13).

More recently, a new generation of fluorescent protein sensors has been developed that only uses one fluorescent protein, produces large changes in fluorescence, and has the potential for multiplexing. Many of these new sensors carry a single, circularly permuted fluorescent protein that converts analyte binding into changes in fluorescence intensity. The green fluorescent GCaMP Ca2+ sensors (14-16), the red R-GECO1 Ca²⁺ sensor (17), the green ElectricPk voltage sensor (18), and the green cGMP sensor (19) use this approach. However, there continues to be a need in the art for additional novel fluorescent sensors that are robust, sensitive, can detect specific analytes and can be used in multiplex systems in real time and in relevant tissues and cell types. The invention of the present application addresses such need.

SUMMARY OF THE INVENTION

The present invention includes a diacylglycerol (DAG) sensor fusion protein comprising a PKC protein comprising a DAG binding domain and a fusion region, and a circularly permuted fluorescent protein, wherein the fusion region is located upstream from the DAG binding domain or within the DAG binding domain; wherein the fluorescent protein is fused with the PKC protein at a fusion site present within the fusion region; and wherein the fluorescence of the DAG sensor fusion protein changes upon binding to DAG.

In some embodiments, the PKC protein may be PKC-δ (delta), PKC-ε (epsilon), PKC-θ (theta), PKC-η (eta), PKC-α (alpha), PKC-β1 (beta 1), PKC-β11 (beta 11), PKC-γ (gamma), or PKC-ξ (zeta). In some embodiments, the PKC protein may be PKC-δ (delta), PKC-ε (epsilon), PKC-θ (theta) or PKC-η (eta). In some embodiments, the PKC protein may be PKC-δ delta.

In some embodiments, the DAG binding domain may comprise a C1 domain. In some embodiments, the fusion region may be located upstream of the C1, or within the C1domain. In some embodiments, the fusion region may comprise additions or deletions of amino acids. In some embodiments, the fusion region may comprise linker sequences.

In some embodiments, the circularly permuted fluorescent protein may comprise a circular permutation in a beta sheet near the chromophore of the fluorescent protein. In some embodiments, the fluorescence of the fusion protein may increase upon binding to DAG. In some embodiments, the fluorescence of the fusion protein may decrease upon binding to DAG.

In some embodiments, the DAG sensor fusion protein may comprise an amino acid sequence that is at least 90% identical to an amino acid sequence selected from SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15, SEQ ID NO:16, SEQ ID NO:17, SEQ ID NO:18, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:21, SEQ ID NO:22, SEQ ID NO:23, SEQ ID NO:24, SEQ ID NO:25, SEQ ID NO:26, SEQ ID NO:27, SEQ ID NO:28, SEQ ID NO:29, SEQ ID NO:30, SEQ ID NO:31, SEQ ID NO:32, SEQ ID NO:33, SEQ ID NO:34, SEQ ID NO:35, and SEQ ID NO:36. In some embodiments, the DAG sensor fusion protein may comprise an amino acid sequence that is at least 90% identical to an amino acid sequence selected from the group SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:12, SEQ ID NO:15, SEQ ID NO:16, SEQ ID NO:17, SEQ ID NO:20, SEQ ID NO:21, SEQ ID NO: 22, SEQ ID NO:24, SEQ ID NO:30.

In some embodiments, the present invention includes a multiplex system for detecting one or more analytes comprising the DAG sensor fusion protein, and one or more additional fluorescent sensors, wherein the additional sensor specifically detects an analyte other than DAG. The additional fluorescent sensor may comprise a fluorescent sensor fusion protein comprising a fluorescent protein, or a fluorescent dye. In some embodiments, the DAG sensor fusion protein may comprise a fluorescent protein that is fluorescent in one region of the spectrum and the additional fluorescent sensor is fluorescent in another region of the spectrum. In some embodiments, the additional fluorescent sensor may be a PiP2 sensor, wherein the fluorescence of the PIP2 sensor changes upon binding to PIP2, or a Calcium sensor, wherein the fluorescence of the Calcium sensor changes upon binding to Calcium, or both.

In some embodiments, the PIP2 sensor may comprise a PLC protein portion that binds to PIP2 and a fluorescent protein.

In some embodiments, the present invention includes a nucleic acid sequence encoding the DAG sensor fusion protein. In some embodiments, it includes a nucleic acid molecule comprising such nucleic acid sequence. In some embodiments, the present invention includes a cell comprising such nucleic acid molecule. In some embodiments, the nucleic acid sequence encoding the DAG sensor fusion protein may be located in the genome of the cell. In some embodiments, the cell may further comprise one or more additional nucleic acid molecules that encode one or more additional fluorescent sensor proteins that specifically detect an analyte other than DAG. In some embodiments, the cell may be a CHO cell, a human Hela cell or a human embryonic kidney (HEK) cell.

In some embodiments, the present invention includes a PIP2 sensor comprising a PLC protein portion that binds to PIP2 and a fluorescent protein, wherein the fluorescence of the PIP2 sensor fusion protein changes upon binding to PIP2. In some embodiments, the fluorescent protein may be a dimerization-dependent fluorescent protein.

In some embodiments, the present invention includes a polypeptide comprising a DAG sensor fusion protein, wherein the DAG sensor fusion protein comprises an amino acid sequence that is at least 90% identical to an amino acid sequence selected from SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15, SEQ ID NO:16, SEQ ID NO:17, SEQ ID NO:18, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:21, SEQ ID NO:22, SEQ ID NO:23, SEQ ID NO:24, SEQ ID NO:25, SEQ ID NO:26, SEQ ID NO:27, SEQ ID NO:28, SEQ ID NO:29, SEQ ID NO:30, SEQ ID NO:31, SEQ ID NO:32, SEQ ID NO:33, SEQ ID NO:34, SEQ ID NO:35, and SEQ ID NO:36. In some embodiments, the present invention includes a nucleic acid sequence encoding such polypeptide.

In some embodiments, the present invention includes a polypeptide comprising an amino acid sequence that is at least 90% identical to an amino acid sequence selected from SEQ ID NO:70 and SEQ ID NO:71. In some embodiments, the present invention includes a nucleic acid sequence encoding such polypeptide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing depicting the domain structure of various isoforms of Protein Kinase C.

FIGS. 2A and 2B. (A) is a schematic diagram, illustrating the design of a prototype of DAG sensor. (B) shows the design of a construct in which a DAG sensor is coupled to a calcium sensor to produce stoichiometrically balanced quantities of each sensor.

FIGS. 3A through 3E show the responses of Green Downward DAG and Upward DAG sensors. (A) Carbachol stimulation of the M1 receptor on cells expressing the Downward DAG sensor produces a 40% loss in fluorescence that occurs over 15 seconds (mean fluorescence over time of 4 cells). (B) The Upward DAG sensor shows a fluorescence increase of 45% over a similar time scale. (C) The signals generated by either sensor return to baseline quite slowly. (D) The apparent EC50 for carbacol-stimulated Upward DAG response is 3.5 uM. (E) The carbachol stimulation does not appear to activate all of the sensor pool in the cell since direct activation of the sensors with a subsequent application of PDBu produces an additional increase in fluorescence.

FIGS. 4A through 4C show that pairing the Green Upward and Downward DAG sensors with R-GECO makes it possible to simultaneously measure DAG and Ca2+ signaling in single cells. (A) The Green Upward DAG sensor response is considerably slower than the red Ca2+ response in response to carbachol stimulation of the M1 receptor. (B) Similar kinetics occur with the Downward DAG sensor. (C) The two sensors can be activated independently: ionomycin, which should raise intracellular Ca2+ without affecting DAG levels produces a change in R-GECO but not Downward DAG, while the subsequent addition of PDBu activates Downward DAG (arrows indicate stimulus artifact). FIG. 4 shows that the response of the Upward DAG-Green sensor to M1 muscarinic acetylcholine receptor (GPCR) activation in living HEK 293 cells occurs in physiologic ranges. The EC50 values for carbachol stimulation are approximately 3 μM.

FIGS. 5A through 5D show that the Green fluorescent sensors Upward DAG2 (A and B) and Downward DAG (C and D) can be co-expressed with the red fluorescent R-GECO1 to simultaneously measure Ca2+ and DAG signaling in living cells. The responses (mean pixel intensity) of individual cells are plotted in A and C. The left axis is green fluorescence (arbitrary units) and the right axis represents red fluorescence. The effect of DMSO on the DAG sensors is negligible at final concentrations of 0.1 to 1%, but detectable at 2% or greater (B and D).

FIGS. 6A through 6D show multiplexing by DAG, PIP2, and Ca2+ sensors. The red PIP2 sensor was coexpressed with the G-GECO1 Ca2 sensor and the M1 receptor. Carbachol addition triggered a simultaneous increase in green fluorescence and decrease in red fluorescence (A & B). To test for interactions between the Ca2+ increase and DAG (C) or PIP2r (D) sensors, ionomycin was added to the culture, followed later by carbachol or PdBU.

FIGS. 7A and 7B show that the PIP2r and DAG sensors can be co-expressed and measured simultaneously. Stimulation of phospholipase C cleaves PIP2 and produces DAG, which is clearly seen in living cells as the red fluorescence of the PIP2r vanishes and the Upward DAG2 sensor increases in fluorescence (A). This is reproducible from cell to cell (B, upper panel). The apparent return to baseline for the Upward DAG2 sensor is considerably faster than the Downward DAG2 or PIP2r sensors, which may be caused photobleaching during the experiment.

FIGS. 8A through 8D show the effect of ATP on activation of the PLC pathway. In HEK cells expressing the human P2Y11 receptor, the addition of ATP (A and C) or UTP (B and D) produces a transient increase in Ca2+ that is consistent with receptor activation. However the simultaneous recording of the Upward (C and D) or Downward (A and B) DAG2 sensors reveals that the ATP is activating the PLC pathway, while UTP is producing a Ca2+ transient through a different pathway.

FIGS. 9A through 9C show that the DAG sensors described here are compatible with automated drug discovery. The Downward DAG2 sensor co-expressed with the M1 (A) or P2Y11 (B) receptor produces a consistent, reproducible signal (Z′>0.6) on a standard fluorescence plate reader. (C) Multiplexing the DAG sensors with R-GECO produces a two dimensional surface on which the negative control wells and positive carbachol responses are unambiguously separated.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed at the design and production of novel fluorescent sensors that can be used in multiplex detection systems and assays. Described herein is the design and construction of a novel Diacyl Glycerol (DAG) sensor that specifically detects intracellular DAG. Further described is a PIP2 sensor that specifically detects intracellular PIP2. Both sensors provide strong fluorescence signals in live cells and can be used in live cell assays in real time, on standard fluorescent plate readers or live cell imaging systems. Additionally, they can be combined with one another, and/or with other sensors, such as a Ca²⁺ sensor, to provide simultaneous detection and measurement of multiple molecules or analytes in multiplex detection systems.

In one aspect, the present invention includes a DAG sensor that can be used to detect changes in DAG concentration in living cells. The DAG sensor of the present invention is based on a design that converts the DAG activation-dependent conformational changes of the PKC protein into a change in fluorescence. Because DAG is involved in many intracellular signaling pathways, including a wide variety of G-Protein Coupled Receptors (GPCRs), the DAG sensor of the present invention is useful in drug discovery, basic research focused on cell signal transduction, and research into the mechanism of diseases associated with DAG dependent signal transduction, such as type II diabetes.

In one embodiment, the DAG sensor comprises a fusion protein comprising a PKC protein containing a DAG binding domain and a fusion region, and a fluorescent protein wherein the fluorescent protein is fused with the PKC protein at a fusion site present within the fusion region and wherein the fluorescence of the DAG sensor fusion protein changes upon binding to DAG.

The term PKC protein refers to the protein Kinase C. A number of isoforms of PKC are known in the art and are encompassed by the present invention. These include, without limitation, conventional PKC isoforms such as a (alpha), β1 (beta 1), β11 (beta 11) and γ (gamma); novel isoforms such as δ (delta), ε (epsilon), θ (theta) and η (eta); as well as atypical isoforms such as the ξ (zeta) isoform. The PKC isoforms have been isolated from a large number of species, including without limitation, drosophila, xenopus, cow, mouse, rat, rabbit, human, etc. The amino acid sequences of these isoforms, as well as the nucleotide sequences of nucleic acid molecules encoding them, are available through public databases such as Genbank and are expressly incorporated herein. FIG. 1 contains a schematic representation depicting a comparison of the domain structures of various PKC isoforms. As shown in FIG. 1, all PKC isoforms contain a regulatory region comprising a pseudosubstrate domain, a DAG binding C1 domain which comprises the subdomains C1a and C1b, and a calcium binding C2 domain; a kinase region comprising an activation loop, and a C terminal (CT) region; and a hinge region that connects the regulatory region and the kinase region. The pseudosubstrate domain lies upstream of the C1 domain. The conventional PKC isoforms, such as α, β1, β11 and γ respond to both DAG and calcium through the binding domains C1 and C2 respectively. The PKC isoforms delta, epsilon, theta and eta contain a novel C2 domain that does not respond to calcium levels, and a C1 domain that has a very high affinity for DAG. The atypical PKC isoform lacks the C2 domain. For example, in the PKC delta isoform represented by SEQ ID NO:1 the pseudosubstrate domain extends from approximately amino acid 140 to amino acid 152. The C1 domain extends from approximately amino acid 158 to 280. The C1 domain comprises the C1a and C1b domains; C1a domain extends from approximately amino acid 158 to 208, while the C1b domain extends from approximately amino acid 230 to 280.

The presence of the novel non-functional C2 and a high affinity C1 domain makes the novel PKC isoforms particularly desirable in the construction of DAG sensors. However, the conventional isoforms can also be used in the construction of DAG sensors by removal or mutation of the C2 domain such that it does not respond to calcium, and the C1 domain can be converted to have a high affinity to DAG with a single mutation (22). Even the low affinity C1 domains of the conventional PKCs can be used to produce a sensor capable of indicating changes in DAG as GFP fused to the C1 domain of the conventional PKC gamma has been shown to translocate in response to DAG signaling (41).

Reference to a protein (or polypeptide) herein includes full-length proteins, fusion proteins, or any fragment, domain, conformational epitope, or homolog of such proteins. As used herein, the term “homolog” is used to refer to a protein or peptide which differs from a naturally occurring protein or peptide (i.e., the “prototype” or “wild-type” protein) by minor modifications to the naturally occurring protein or peptide, but which maintains the basic protein and side chain structure of the naturally occurring form. Such changes include, but are not limited to: changes in one or a few amino acid side chains; changes one or a few amino acids, including deletions (e.g., a truncated version of the protein or peptide) insertions and/or substitutions; changes in stereochemistry of one or a few atoms; and/or minor derivatizations, including but not limited to: methylation, glycosylation, phosphorylation, acetylation, myristoylation, prenylation, palmitation, amidation. A homolog can have either enhanced, decreased, or substantially similar properties as compared to the naturally occurring protein or peptide. Homologs can be produced using techniques known in the art for the production of proteins including, but not limited to, direct modifications to the isolated, naturally occurring protein, direct protein synthesis, or modifications to the nucleic acid sequence encoding the protein using, for example, classic or recombinant DNA techniques to effect random or targeted mutagenesis. A homolog of a given protein may comprise, consist essentially of, or consist of, an amino acid sequence that is at least about 45%, or at least about 50%, or at least about 55%, or at least about 60%, or at least about 65%, or at least about 70%, or at least about 75%, or at least about 80%, or at least about 85%, or at least about 90%, or at least about 95% identical, or at least about 95% identical, or at least about 96% identical, or at least about 97% identical, or at least about 98% identical, or at least about 99% identical (or any percent identity between 45% and 99%, in whole integer increments), to the amino acid sequence of the reference protein. In one embodiment, the homolog comprises, consists essentially of, or consists of, an amino acid sequence that is less than 100% identical, less than about 99% identical, less than about 98% identical, less than about 97% identical, less than about 96% identical, less than about 95% identical, and so on, in increments of 1%, to less than about 70% identical to the naturally occurring amino acid sequence of the reference protein.

Accordingly, the term PKC protein includes a full length PKC isoform, or truncated or mutated versions of it that contain, at a minimum, a DAG binding domain and a fusion region. The term DAG binding domain refers to a portion or region of the PKC protein that is capable of binding to DAG. In some embodiments, the term DAG binding domain refers to the full C1 domain of the PKC. In some embodiments, the DAG binding domain refers to a truncated or mutated version of the C1 domain or a fragment of the C1 domain, such as C1a or C1b or fragments thereof, that maintains the ability to bind to DAG.

The term fusion region refers to a region of the PKC protein which contains the fusion sites at which the fluorescent protein is inserted. Without wishing to be bound by theory it is believed that the binding of the C1 domain to DAG leads to large conformational changes in the fusion region of the PKC protein, which in turn alters the chromophore environment of the fluorescent protein, thereby producing a change in the fluorescence.

In some embodiments the fusion region lies upstream of the DAG binding domain. In some embodiments it is located in the region between the pseudosubstrate domain and the C1 domain. In some embodiments it is located within the DAG binding domain. In some embodiments it is located within the C1 domain. In some embodiments it is located within the C1 domain, upstream of the C1b domain. In some embodiments it is located in the C1a domain or the region between C1a and C1b domains. In some embodiments, it is located downstream of the DAG binding domain. In some embodiments it is located in the hinge region that lies between the C1 domain and the kinase domain.

In some embodiments the fusion region comprises the native amino acid sequence of the PKC protein, and the fluorescent protein is inserted at fusion sites within the native PKC protein sequence. In some embodiments the fusion region may comprise additions or deletions of amino acids that make the sequence deviate from the native sequence. For example, in various embodiments, the fusion region may comprise addition of at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20 amino acids to the native sequence. In various embodiments, the fusion region may comprise deletions of at least 1, 2, 3, 4, 5, 6, 7, 8 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, or 22 amino acids from the native sequence. In some embodiments the fusion region may comprise conservative substitutions of the amino acids of the native sequence. For example, as shown in Table 2 in some embodiments the fusion region comprises the region between amino acid position about 150 to amino acid position about 173, and may further comprise additions or deletions of amino acids to this region.

In some embodiments the fusion region further comprises linker sequences that may be present at the N terminal and/or C terminal ends of the circularly permuted fluorescence protein and that link the circularly permuted protein to the PKC protein. Linkers containing amino acids with side chains that give the linker ridged structure are particularly important to converting the conformational changes of the PKC to changes in the structure of the fluorescent protein barrel. Similarly, linkers with bulky amino acids that can form a surface/structure capable of occluding the hole in the side of the barrel produced by circular permutation are best capable of producing large changes in fluorescence by protecting the chromophore environment in one configuration and in another configuration producing a large hole in the side of the protein barrel that renders the chromophore less fluorescent. The linker sequences may be at least 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 amino acids long. Examples of such linkers include, without limitation, LE, AI, PV, SH, TR, FN, or ENNHLS. In some embodiments the linker sequence may comprise the sequence LE or TR. It is well known that minor adjustments in the linkers interconnecting the circularly permuted fluorescent protein and the analyte-sensing domains can have a large impact on the amplitude of the fluorescence change (14, 15).

Example 1 describes in detail the design and construction of DAG sensors using PKC delta [SEQ ID NO:1]. Briefly, a series of genetically encoded, fluorescent DAG sensors were constructed. Sixty four candidates were produced that fused a circularly permuted green fluorescent protein [SEQ ID NO:2] to full length or truncated PKCδ. See Table 1. Two robust prototype sensors called Upward DAG (G17.2B, SEQ ID NO:4) (in which fluorescence increases upon binding to DAG) and Downward DAG (G23, SEQ ID NO:17) (in which fluorescence decreases upon binding to DAG) were recovered from this initial effort (20). In these two sensors, the circularly permuted green fluorescent protein and the linker were positioned either between the pseudosubstrate and the C1 domain of PKCδ or after the first amino acid of the C1 domain. An additional 156 variants of the original Upward and Downward DAG sensors were created and a total of thirty sensors were recovered. See Table 2.

The complete amino acid sequences of these sensors are represented by SEQ ID NO:4 (G17.2B, also referred to as Upward DAG), SEQ ID NO:5 (G17-18, also referred to as Upward DAG2), SEQ ID NO:6 (G17-19), SEQ ID NO:7 (G18-20), SEQ ID NO:8 (G19-17), SEQ ID NO:9 (G19-18), SEQ ID NO:10 (G19-20), SEQ ID NO:11 (G20-17), SEQ ID NO:12 (G20-28, also referred to as Downward DAG2), SEQ ID NO:13 (G21-17), SEQ ID NO:14 (G21-19), SEQ ID NO:15 (G21-20), SEQ ID NO:16 (G21-23), SEQ ID NO:17 (G23, also referred to as Downward DAG), SEQ ID NO:18 (G23-18), SEQ ID NO:19 (G23-19), SEQ ID NO:20 (G27-19), SEQ ID NO:21 (G27-22), SEQ ID NO:22 (G28-18), SEQ ID NO:23 (G28-27), SEQ ID NO:24 (G29-18), SEQ ID NO:25 (G29-23), SEQ ID NO:26 (G29-24), SEQ ID NO:27 (G30-21), SEQ ID NO:28 (G19-30), SEQ ID NO:29 (G21-30), SEQ ID NO:30 (G23-30), SEQ ID NO:31 (G24-30), SEQ ID NO:32 (G28-30), SEQ ID NO:33 (G29-30).

The nucleotide sequences encoding these sensors are provided in SEQ ID NO:37, SEQ ID NO:38, SEQ ID NO:39, SEQ ID NO:40, SEQ ID NO:41, SEQ ID NO:42, SEQ ID NO:43, SEQ ID NO:44, SEQ ID NO:45, SEQ ID NO:46, SEQ ID NO:47, SEQ ID NO:48, SEQ ID NO:49, SEQ ID NO:50, SEQ ID NO:51, SEQ ID NO:52, SEQ ID NO:53, SEQ ID NO:54, SEQ ID NO:55, SEQ ID NO:56, SEQ ID NO:57, SEQ ID NO:58, SEQ ID NO:59, SEQ ID NO:60, SEQ ID NO:61, SEQ ID NO:62, SEQ ID NO:63, SEQ ID NO:64, SEQ ID NO:65, SEQ ID NO:66, respectively.

The details of the fusion region and the specific fusion sites are presented in Tables 1 and 2. The amino acid positions detailed in Example 1 and Tables 1 and 2, are in reference to SEQ ID NO:1.

Example 1 further describes the construction of three additional DAG sensors in which a circularly permuted red fluorescent protein (cpMapple) [SEQ ID NO:3] was fused to a truncated PKCδ. The complete amino acid sequences of these sensors are represented by SEQ ID NO:34 (R17-2b), SEQ ID NO:35 (R19), and SEQ ID NO:36 (R20). The nucleotide sequences encoding these sensors are provided in SEQ ID NO:67, SEQ ID NO:68, SEQ ID NO:69, respectively.

Thus, described herein are novel DAG sensor proteins comprising an amino acid sequence represented by SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15, SEQ ID NO:16, SEQ ID NO:17, SEQ ID NO:18, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:21, SEQ ID NO:22, SEQ ID NO:23, SEQ ID NO:24, SEQ ID NO:25, SEQ ID NO:26, SEQ ID NO:27, SEQ ID NO:28, SEQ ID NO:29, SEQ ID NO:30, SEQ ID NO:31, SEQ ID NO:32, SEQ ID NO:33, SEQ ID NO:34, SEQ ID NO:35, and SEQ ID NO:36.

In some embodiments, the amino acid sequences consist or consist essentially of any of the aforementioned SEQ ID NOs. In some embodiments, the amino acid sequences comprise any of the aforementioned SEQ ID NOs., optionally with one or more conservative amino acid substitutions (e.g., with 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or a range between any two of the aforementioned numbers, or more than twenty conservative amino acid substitutions, so long as the desired function of the sensor is maintained (i.e. substantially maintained). Conservative substitutions typically include substitutions within the following groups: glycine and alanine; valine, isoleucine and leucine; aspartic acid, glutamic acid, asparagine, and glutamine; serine and threonine; lysine and arginine; and phenylalanine and tyrosine. Substitutions may also be made on the basis of conserved hydrophobicity or hydrophilicity (Kyte and Doolittle, J. Mol. Biol. 157:105 (1982)), or on the basis of the ability to assume similar polypeptide secondary structure (Chou and Fasman, Adv. Enzymol. 47: 45 (1978)), or tertiary or quaternary structures. In some embodiments, the number of amino acid substitutions in the sequences may be expressed as a percentage of the total number of amino acids present. For example, about 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 4.5%, 5.0%, 5.5%, 6.0%, 6.5%, 7%, 7.5%, 8.0%, 8.5%, 9.0%, 9.5%, 10.0%, 15%, 20%, 25%, 30%, 40%, 50%, or a range between any two of the aforementioned numbers, of the amino acids present can be substituted with a conservative amino acid(s), so long as the desired function of the sensor is substantially maintained. Also included are amino acid sequences that possess at least about 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99 percent or more identity to any of the aforementioned SEQ ID NOs., so long as the desired function of the sensor is substantially maintained.

Further described herein are nucleic acid sequences that encode the aforementioned DAG sensor proteins. In some embodiments these are represented by SEQ ID NO:37, SEQ ID NO:38, SEQ ID NO:39, SEQ ID NO:40, SEQ ID NO:41, SEQ ID NO:42, SEQ ID NO:43, SEQ ID NO:44, SEQ ID NO:45, SEQ ID NO:46, SEQ ID NO:47, SEQ ID NO:48, SEQ ID NO:49, SEQ ID NO:50, SEQ ID NO:51, SEQ ID NO:52, SEQ ID NO:53, SEQ ID NO:54, SEQ ID NO:55, SEQ ID NO:56, SEQ ID NO:57, SEQ ID NO:58, SEQ ID NO:59, SEQ ID NO:60, SEQ ID NO:61, SEQ ID NO:62, SEQ ID NO:63, SEQ ID NO:64, SEQ ID NO:65, SEQ ID NO:66, SEQ ID NO:67, SEQ ID NO:68, and SEQ ID NO:69.

In some embodiments, the nucleic acid sequence can consist or consist essentially of any of the aforementioned SEQ ID NOs. Also provided are nucleic acid sequences that possess at least about 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99 percent or more identity to any of the aforementioned SEQ ID NOs. Further included are nucleic acid molecules that hybridize to, or are the complements of the aforementioned molecules. Nucleic acids that encode the sensors having stated amino acid sequences, as well as variants, and fragments thereof are also included. These sequences include all degenerate sequences related to a specific amino acid sequence, i.e., all nucleic acids having a sequence that encodes one particular polypeptide sequence as well as all nucleic acids, including degenerate nucleic acids, encoding the disclosed variants and derivatives of the polypeptide sequences. Thus, while each particular amino acid and nucleic acid sequence may not be written out herein, it is understood that each and every sequence is in fact disclosed and described herein through the disclosed polypeptide sequences.

It is noted that while the present invention is exemplified with the protein PKC-delta, the disclosure is applicable to other PKC isoforms and based on the disclosure of this application, one skilled in the art will be readily able to construct and use DAG sensors using other isoforms. For example, the novel PKC isoforms ε (epsilon), θ (theta) and η (eta) have similar structure, sequence, and binding properties as the delta isoform, and may be substituted for the delta isoform. The conventional PKC isoforms, such as α, β1, β11 and γ which respond to both DAG and calcium through binding domains C1 and C2 respectively, may be used in the construction of a DAG sensor by removal or mutation of the C2 domain such that it does not respond to calcium; additionally, their C1 domain can be converted to have a high affinity to DAG.

The DAG sensor further comprises a circularly permuted fluorescent protein. In a circularly permuted fluorescent protein the N and the C termini of the protein are placed adjacent to the chromophore. A number of fluorescent proteins are known in the art and may be circularly permuted to be used in the construction of the sensor of the present invention. The examples of these include without limitation, green fluorescent protein (GFP), and its variants such as red fluorescent protein, yellow fluorescent protein, enhanced green fluorescent protein (EGFP), enhanced yellow fluorescent protein (EYFP), Emerald, mApple, mPlum, mCherry, tdTomato, mStrawberry, J-Red, DsRed-monomer, mOrange, MKO, mCitrine, Venus, YPet, CyPet, mCFPm, Cerluean and T-Sapphire. Such fluorescent proteins are discussed in Shaner et al., A guide to choosing fluorescent proteins, Nature methods, 2:12, 905-909 (2005), and are expressly incorporated herein. Additional examples of fluorescent proteins include, mKOK, mUKG (44), Clover, Ruby (45), epFP650, epFP670 (46), mKate (47), tagRFP (48), mRagGFP, mTAgBFP and EBFP2 (49). A number of circularly permuted fluorescent proteins are described in the art and may be used in the present invention. (Baird et al., Nagai et al, Nakai et al, Shui et al).

Fluorescent proteins typically exhibit a beta barrel structure containing the chromophore. For example, green fluorescent protein consists of eleven β-sheets that form a barrel shaped structure with six alpha helices containing the covalently bonded chromophore 4-(p-hydroxybenzylidene)imidazolidin-5-one (HBI) running through the center. This barrel of eleven β-sheets protects the cell from the chromophore, isolates the chromophore from the environment around the protein, and most importantly stabilizes the chromophore in a relatively ridged conformation that makes the chromophore fluorescent. When analyte sensing domains are fused to the original N- and C-termini of the fluorescent protein, their movements do not produce changes in fluorescence. However, when the original N- and C-termini are fused with a short linker, and new N- and C-termini are introduced in the middle of one of the beta sheets of the barrel, a circularly permuted fluorescent protein is produced with new properties. Analyte sensing domains fused to these new termini can produce very large changes in fluorescence. Without wishing to be bound by theory, it is believed that circular permutation in a beta sheet of the barrel near the chromophore allows a sensor to create a difference in the chromophore environment, thereby producing a change in fluorescence. Thus, for instance in the Calcium sensor GCaMP3 (described in United States Patent Application 20120034691), the calcium binding domains are placed adjacent to the chromophore of the circularly permuted green fluorescent protein. In one conformation there is an opening in the side of the beta barrel of the fluorescent protein and the chromophore is solvent accessible. When the binding domains move in response to activation by calcium, the hole is closed, and the new environment of the chromophore causes it to become fluorescent.

As exemplified herein, in some embodiments, the sensor comprises a circularly permuted green fluorescent protein described in Zhao 2011 (17). This version comprises EGFP circularly permuted around amino acids 149-144 [SEQ ID NO:2]. In another embodiment, the sensor comprises a circularly permuted red fluorescent protein (Mapple) [SEQ ID NO:3]. The circularly permuted red fluorescent protein is not analogous to the circularly permuted green fluorescent protein, indicating that different fluorescent proteins can be circularly permuted in different places and used effectively in the sensors of the present invention.

The fluorescence of the DAG sensor may increase upon binding of the DAG to the sensor, as in the Upward DAG and Upward DAG2 sensors, or decrease upon binding of the DAG to the sensor, as in the Downward DAG and Downward DAG2 sensors. Such properties of the sensors are indicated in Table 2.

Further described herein is a novel sensor that detects Phosphatidylinositol 4,5-bisphosphate or PIP₂. Phospholipase C hydrolyses PIP2 to produce DAG and IP3. The PIP2 sensor was created by fusing the pleckstrin homology (PH) domain of PLCδ to two different components of the recently described dimerization-dependent red fluorescent proteins (26). The Pleckstrin homology domain, roughly 100 amino acids in size, is a feature found in many proteins (42). In the case of the phospholipse C delta, the PH domain of the protein specifically binds to PIP2 (43). Previous work has shown that the translocation of the PLCδ PH domain can be used to measure PIP2 turnover (27), and if the PH domains carry FRET pairs of fluorescent proteins a small change in FRET occurs when PLC is activated (28). To create a more robust sensor that does not involve FRET, and which produces a larger signal with a single fluorescent protein, we fused the PH domain to each member of the ddRFP pair. The design and construction of the PIP2 sensor is described in Example 4.

One advantage of sensors constructed with single fluorescent proteins is that they use less of the visible spectrum than FRET-based systems. This means that different sensors of different colors can be combined to monitor multiple signaling pathways simultaneously. The sensors described in the present application produce large changes in fluorescence that can be readily detected even on simple fluorescent plate readers. Because the sensors are based upon single fluorescent proteins, they can be readily multiplexed with other fluorescent protein based sensors, including without limitation, the calcium sensor R-GECO1 described previously in Zhao et al. 2011 (17), the R-CAMP sensor who structure has been described and deposited with the Protein Data Bank DOI:10.2210/pdb3u0k/pdb or the PIP2 sensor described herein. They can also be used with fluorescent dyes. A number of such dyes are known and available commercially. These include without limitation, voltage sensitive membrane dyes such as Di-4-ANEPPS, and Di-8ANEPPS. They could also be used with Ca2⁺ indicator dyes such as Fluo-3, Fluo-4, Rhod-2, Oregon Green, Calcium Green, Calcium Orange, Calcium Crimson, Fura Red or Calcein. They could also be used with bioluminescent reporters, such as cAMP-Glo (commercially available from Promega). In multiplex assays the different sensor proteins may be encoded by different expression vectors and coexpressed, or may be coupled to produce stoichiometrically balanced quantities of each sensor. Examples 2-7 contain examples of various multiplex assays.

Such multiplexing improves the quality of the information produced in a screen in several ways. First, the simultaneous detection of multiple components of a signaling pathway provides an unambiguous read-out for a particular pathway. Second, detection of two different signals can be used to improve assay performance/reliability. Finally, the use of multiplex sensors such as these have the potential to provide new views of agonist-biased signaling by providing relative ratios of the activity of different signaling components (8). The multiplex sensors described here offer new opportunities for live cell assays by producing large, reproducible changes in fluorescence that can be detected on standard fluorescence plate readers used in laboratory automation. These live cell assays require no additional reagents, cell lysis, or complex liquid handling steps. The sensors are ready for routine use on standard equipment, and even better signals can be obtained with plate readers that can measure the response of the sensors in every well over time. The advent of multiplex sensors for both Ca2+ and cAMP for example (2, 4), shows that cells can produce anti-phase, cyclic patterns of signaling that can only be detected by collecting the responses of the two sensors over time. Similarly, the Ca2+ and DAG/PIP2 responses shown herein in examples 2-7 are quite different, with different rates of onset and return to baseline. As further demonstrated in Example 8, these interesting and biologically relevant patterns of signaling can be captured in microplate format, by measuring multiple signals over several time points at 0.1 to 5 Hz, with the Molecular Devices Fluorescent Imaging Plate Reader (FLIPR) and Hamamatsu FDSS (31, 32).

Also provided herein are vectors comprising the sensor-encoding nucleic acid sequences. Examples of suitable vectors include, but are not limited to, plasmids, artificial chromosomes, such as BACs, YACs, or PACs, and viral vectors. As used herein, vectors are agents that transport the disclosed nucleic acids into a cell without degradation and, optionally, include a promoter yielding expression of the nucleic acid molecule in the cells into which it is delivered.

Viral vectors are, for example, Adenovirus, Adeno-associated virus, herpes virus, Vaccinia virus, Polio virus, Sindbis, and other RNA viruses, including these viruses with the HIV backbone. Any viral families which share the properties of these viruses which make them suitable for use as vectors are suitable. Retroviral vectors, in general are described by Coffin et al., Retorviruses, Cold Spring Harbor Laboratory Press (1997), which is incorporated by reference herein for the vectors and methods of making them. The construction of replication-defective adenoviruses has been described (Berkner et al., J. Virology 61:1213-20 (1987); Massie et al., Mol. Cell. Biol. 6:2872-83 (1986); Haj-Ahmad et al., J. Virology 57:267-74 (1986); Davidson et al., J. Virology 61:1226-39 (1987); Zhang et al., BioTechniques 15:868-72 (1993)). Recombinant adenoviruses have been shown to achieve high efficiency after direct, in vivo delivery to airway epithelium, hepatocytes, vascular endothelium, CNS parenchyma, and a number of other tissue sites. Other useful systems include, for example, replicating and host-restricted non-replicating vaccinia virus vectors.

Non-viral based vectors, can include expression vectors comprising nucleic acid molecules and nucleic acid sequences encoding polypeptides, wherein the nucleic acids are operably linked to an expression control sequence. Suitable vector backbones include, for example, those routinely used in the art such as plasmids, artificial chromosomes, BACs, YACs, or PACs. Numerous vectors and expression systems are commercially available from such corporations as Novagen (Madison, Wis.), Clonetech (Pal Alto, Calif.), Stratagene (La Jolla, Calif.), and Invitrogen/Life Technologies (Carlsbad, Calif.). Vectors typically contain one or more regulatory regions. Regulatory regions include, without limitation, promoter sequences, enhancer sequences, response elements, protein recognition sites, inducible elements, protein binding sequences, 5′ and 3′ untranslated regions (UTRs), transcriptional start sites, termination sequences, polyadenylation sequences, and introns. Preferred promoters controlling transcription from vectors in mammalian host cells may be obtained from various sources, for example, the genomes of viruses such as polyoma, Simian Virus 40 (SV40), adenovirus, retroviruses, hepatitis B virus, and most preferably cytomegalovirus (CMV), or from heterologous mammalian promoters, e.g. .beta.-actin promoter or EF1.alpha. promoter, or from hybrid or chimeric promoters (e.g., CMV promoter fused to the .beta.-actin promoter). Promoters from the host cell or related species are also useful herein. Enhancer generally refers to a sequence of DNA that functions at no fixed distance from the transcription start site and can be either 5′ or 3′ to the transcription unit. Furthermore, enhancers can be within an intron as well as within the coding sequence itself. They are usually between 10 and 300 base pairs in length, and they function in cis. Enhancers usually function to increase transcription from nearby promoters. Enhancers can also contain response elements that mediate the regulation of transcription. While many enhancer sequences are known from mammalian genes (globin, elastase, albumin, fetoprotein, and insulin), typically one will use an enhancer from a eukaryotic cell virus for general expression. Preferred examples are the SV40 enhancer on the late side of the replication origin, the cytomegalovirus early promoter enhancer, the polyoma enhancer on the late side of the replication origin, and adenovirus enhancers.

The promoter and/or the enhancer can be inducible (e.g. chemically or physically regulated). A chemically regulated promoter and/or enhancer can, for example, be regulated by the presence of alcohol, tetracycline, a steroid, or a metal. A physically regulated promoter and/or enhancer can, for example, be regulated by environmental factors, such as temperature and light. Optionally, the promoter and/or enhancer region can act as a constitutive promoter and/or enhancer to maximize the expression of the region of the transcription unit to be transcribed. In certain vectors, the promoter and/or enhancer region can be active in a cell type specific manner. Optionally, in certain vectors, the promoter and/or enhancer region can be active in all eukaryotic cells, independent of cell type. Preferred promoters of this type are the CMV promoter, the SV40 promoter, the .beta.-actin promoter, the EFLalpha. promoter, and the retroviral long terminal repeat (LTR).

Cells comprising the sensors of the present invention, the sensor-encoding nucleic acid sequences or vectors comprising the sensor-encoding nucleic acid sequence are provided. The cell can be, for example, a eukaryotic or prokaryotic cell. Suitable cells include, but are not limited to cells of E. coli, Pseudomonas, Bacillus, Streptomyces; fungi cells such as yeasts (Saccharomyces, and methylotrophic yeast such as Pichia, Candida, Hansenula, and Torulopsis); and animal cells, such as CHO, R1.1, B-W and LM cells, African Green Monkey kidney cells (for example, COS 1, COS 7, BSC1, BSC40, and BMT10), insect cells (for example, Sf9), human cells and plant cells. Suitable human cells include, for example, HeLa cells or human embryonic kidney (HEK) cells. Cells that can be used herein are commercially available from, for example, the American Type Culture Collection (ATCC), P.O. Box 1549, Manassas, Va. 20108. See also F. Ausubel et al., Current Protocols in Molecular Biology, John Wiley & Sons, New York, N.Y., (1998). Optionally, the sensor-encoding nucleic acid sequence may be located in the genome of the cell.

Methods of making the provided cells are known and the method of transformation and choice of expression vector will depend on the host system selected. Transformation and transfection methods are described, e.g., in F. Ausubel et al., Current Protocols in Molecular Biology, John Wiley & Sons, New York, N.Y., (1998), and, as described above, expression vectors may be chosen from examples known in the art. There are a number of compositions and methods which can be used to deliver the nucleic acid molecules and/or polypeptides to cells, either in vitro or in vivo via, for example, expression vectors. These methods and compositions can largely be broken down into two classes: viral based delivery systems and non-viral based deliver systems. Such methods are well known in the art and readily adaptable for use with the compositions and methods described herein.

As used herein, unless otherwise specified, reference to a percent (%) identity refers to an evaluation of homology which is performed using: (1) a BLAST 2.0 Basic BLAST homology search using blastp for amino acid searches and blastn for nucleic acid searches with standard default parameters, wherein the query sequence is filtered for low complexity regions by default (described in Altschul, S. F., Madden, T. L., Schääffer, A. A., Zhang, J., Zhang, Z., Miller, W. & Lipman, D. J. (1997) “Gapped BLAST and PSI-BLAST: a new generation of protein database search programs.” Nucleic Acids Res. 25:3389-3402, incorporated herein by reference in its entirety); (2) a BLAST 2 alignment (using the parameters described below); (3) and/or PSI-BLAST with the standard default parameters (Position-Specific Iterated BLAST. It is noted that due to some differences in the standard parameters between BLAST 2.0 Basic BLAST and BLAST 2, two specific sequences might be recognized as having significant homology using the BLAST 2 program, whereas a search performed in BLAST 2.0 Basic BLAST using one of the sequences as the query sequence may not identify the second sequence in the top matches. In addition, PSI-BLAST provides an automated, easy-to-use version of a “profile” search, which is a sensitive way to look for sequence homologues. The program first performs a gapped BLAST database search. The PSI-BLAST program uses the information from any significant alignments returned to construct a position-specific score matrix, which replaces the query sequence for the next round of database searching. Therefore, it is to be understood that percent identity can be determined by using any one of these programs.

According to the present invention, the terms “modification” and “mutation” can be used interchangeably, particularly with regard to the modifications/mutations to the primary amino acid sequences of a protein or peptide (or nucleic acid sequences) described herein. The term “modification” can also be used to describe post-translational modifications to a protein or peptide or, for example, complexing a protein or peptide with another compound or tethering the protein, such as by a glycerophosphatidyl inositol (GPI) anchor. Such modifications can be considered to be mutations, for example, if the modification is different than the post-translational modification that occurs in the natural, wild-type protein or peptide.

The practice of the present invention will employ, unless otherwise indicated, conventional techniques of molecular biology (including recombinant techniques), microbiology, cell biology, biochemistry, nucleic acid chemistry, and immunology, which are well known to those skilled in the art. Such techniques are explained fully in the literature, such as, Methods of Enzymology, Vol. 194, Guthrie et al., eds., Cold Spring Harbor Laboratory Press (1990); Biology and activities of yeasts, Skinner, et al., eds., Academic Press (1980); Methods in yeast genetics: a laboratory course manual, Rose et al., Cold Spring Harbor Laboratory Press (1990); The Yeast Saccharomyces: Cell Cycle and Cell Biology, Pringle et al., eds., Cold Spring Harbor Laboratory Press (1997); The Yeast Saccharomyces: Gene Expression, Jones et al., eds., Cold Spring Harbor Laboratory Press (1993); The Yeast Saccharomyces: Genome Dynamics, Protein Synthesis, and Energetics, Broach et al., eds., Cold Spring Harbor Laboratory Press (1992); Molecular Cloning: A Laboratory Manual, second edition (Sambrook et al., 1989) and Molecular Cloning: A Laboratory Manual, third edition (Sambrook and Russel, 2001), (jointly referred to herein as “Sambrook”); Current Protocols in Molecular Biology (F. M. Ausubel et al., eds., 1987, including supplements through 2001); PCR: The Polymerase Chain Reaction, (Mullis et al., eds., 1994); Harlow and Lane (1988) Antibodies, A Laboratory Manual, Cold Spring Harbor Publications, New York; Harlow and Lane (1999) Using Antibodies: A Laboratory Manual Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (jointly referred to herein as “Harlow and Lane”), Beaucage et al. eds., Current Protocols in Nucleic Acid Chemistry John Wiley & Sons, Inc., New York, 2000); Casarett and Doull's Toxicology The Basic Science of Poisons, C. Klaassen, ed., 6th edition (2001), and Vaccines, S. Plotkin and W. Orenstein, eds., 3rd edition (1999).

EXAMPLES Example 1 Illustrates the Design and Construction of the DAG Biosensors

Sixty four different prototypes of a DAG sensor were created by fusing a circularly permuted enhanced green fluorescent protein from the calcium sensor G-GECO (cpEGFP) previously described in Zhao et al. 2011 to 30 different positions within the PKC-delta isoform. The sequence of the cpGFP is represented by SEQ ID NO:2 and the full length sequence of the PKC-delta isoform is represented by SEQ ID NO:1. As detailed in table 1, cpEGFP was inserted at various fusion sites in the full length PKC-delta, or fragments of PKC delta comprising N terminal truncation. Additionally, some of the constructs contained a deletion in the region immediately adjacent to the cpGFP insertion site. The length of the deletions ranged from 1 amino acid to 68 amino acids.

PCR amplification was used to generate fragments of PKC-delta and cpEGFP of G-GECO. Different combinations of PKC fragments were then paired with the cpEGFP amplicon and cloned into a modified version of the mammalian expression vector pcDNA3.1 using the In-Fusion Cloning system (Clonetech Laboratories Inc, Mountain View, Calif.). The pcDNA3.1 vector was obtained from Life Technologies (Grand Island, N.Y.). As detailed in table 1, thirty two of the prototypes involved inserting the cpEGFP into the full length PKC delta, and an additional 32 constructs were created in which the N-terminal region of PKC delta containing the C2 domain was deleted.

To test the functionality of the 64 fusion proteins, each construct was coexpressed with the M1 acetylcholine receptor, which couples to the Gq signaling pathway, in HEK 293 cells, and the fluorescence measured as described below.

HEK 293 cells (21) were cultured in EMEM supplemented with 10% fetal bovine serum and Penicillin-Streptomycin at 37° C. in 5% CO2. The cells and Eagle's Minimum Essential Medium (EMEM) were purchased from ATCC (Manassas, Va.). Prior to cell seeding, 96-well glass-bottom plates were coated with Poly-D-Lysine. Cells were seeded on the plates, transfected using Lipofectamine 2000 Transfection Reagent according to the manufacturer's protocol, and incubated for 24-48 hours at 37° C. in 5% CO₂. 60 ng of sensor DNA was co-transfected with 40 ng of human M1 muscarinic acetylcholine receptor per well. Pen-Strep liquid and Lipofectamine 2000 were obtained from Life Technologies (Grand Island, N.Y.). Poly-D-Lysine was purchased from Fisher Scientific (Pittsburgh, Pa.).

EMEM culture medium was replaced with 1×DPBS prior to screening transfected cells for fluorescence. A Zeiss Axiovert S 100TV inverted microscope equipped with computer controlled excitation/emission filter wheels, shutters, and a Qimaging Retiga Exi CCD camera (Surrey, BC Canada) was used to image cells at 25° C. using the 10× objective lens. 480±20 nm excitation and 535±25 nm emission filters were used resolve the green fluorescence from the DAG sensors, and 572±20 nm and 630±30 nm filters were used to collect the R-GECO signal. Cells were analyzed for increases or decreases in fluorescence intensity upon addition of Carbachol, PdBU, DMSO or Ionomycin. To analyze the image stacks, background fluorescence was defined as a region of the image that contained no cells. The average value of this region was subtracted frame by frame from the measurements of the mean pixel values of the fluorescent cells. Fluorescence intensity data was plotted and analyzed with IGOR (Wavemetrics, Oswego Ore.).

For transient expression and screening in an automated fluorescence plate reader, HEK 293T cells were cultured in Corning Co-Star Polystyrene 96 well plates coated with Poly-D-Lysine. HEK293T cells were plated at 35,0000 cells/well in 100 μl growth medium per well without antibiotics so that the cells would be 90-95% confluent at the time of transfection (approximately 24 hours later). For each transfection (i.e. one well in a 96-well plate), 160 ng of plasmid DNA (120 ng sensor+40 ng receptor) was diluted in 25 μl of Opti-MEM, 0.48 ul of lipofectamine 2000 was diluted in 25 μl of Opti-MEM, and these were then mixed and added to the cells. Cells were incubated in this mixture for 4 to 6 hours, and then the mixture was replaced with fresh medium. Prior to scanning a plate on the Biotek Synergy Mx, EMEM culture medium was replaced with 250 μl of 1×DPBS per well. Plates were read at 25° C., using monochromators set to 488/20 nm excitation and 530/20 nm emission to resolve the green fluorescence from the DAG sensor.

Two robust prototype sensors, Upward DAG and Downward DAG were recovered from this initial effort (20). Application of carbachol, an agonist of the M1 acetylcholine receptor, produced a remarkable a 45% increase in fluorescence in the Green Upward DAG sensor (FIG. 3B) and 40% decrease in fluorescence in the Green Downward DAG sensor (FIG. 3A). These changes were easily detected in time-lapse imaging and occurred in all transfected cells with remarkably little cell to cell variability. The increase or decrease of the signal produced by the Upward or Downward DAG, respectively, was reasonably fit by a single exponential function with a time constant of 6 to 11 seconds. The signals then returned to baseline quite slowly (t−170 seconds, FIG. 3C).

Both the Upward and Downward DAG sensors showed robust changes in fluorescence that are an order of magnitude larger than the previously reported, FRET-based DAG sensors. In transient expression it is possible to produce high concentrations of the protein-based sensor than the analyte itself [Falkenburger et al]. To test whether our measurements of the maximal sensor responses might be an underestimate, cells were first stimulated with carbachol and then the phorbol ester PDBu was added to directly activate any remaining sensors within the cell (FIG. 3D). This produced an additional doubling of the change in intensity, indicating that not all of the sensors in a given cell were activated by the carbachol, and that larger changes in fluorescence might be seen at lower intracellular concentrations of sensor, such as in the context of stable cell lines or transgenic animals.

Surprisingly, one sensor increases fluorescence as a result of activation (Upward DAG), while an insertion only 6 amino acids away produced a sensor that decreases fluorescence as a result of activation (Downward DAG). To our knowledge, this is the first example of small change in the position of the fluorescent protein producing an inversion of the signal produced by the sensor.

To optimize these prototype sensors, we created an additional 156 variants of the original Upward DAG and Downward DAG sensors, which helped identify additional 28 DAG sensors which produce large responses. The design of all thirty sensors is summarized in Table 2. As detailed in table 2, all sensors contain an N-terminal truncation of PKC delta that eliminates the C2 domain. The truncation was at L122. Thus, all sensors contain E123-Q150, with the table 2 beginning at Q150. Left side of the table lists the PKC delta protein sequence upstream of the inserted cpEGFP (see central green column)—In a given row, the final amino acid before the green column indicates the amino acid of PKC delta after which the cpEGFP is inserted. In all but one of these sensors, the cpEGFP and the linker were either positioned between the pseudosubstrate domain and the C1 domain of PKCδ, or in the C1a domain of PKCδ. Note that the PcpG30-21 sensor does not follow the general organization of the table. In this sensor, the insertion site of cpEGFP was well upstream of the insertion sites in the rest of the sensors. The amino acid after which cpEGFP was inserted was E134. Thus, this sensor is missing D135-H154 of the wild type PKC.

Similarly DAG sensors comprising a red fluorescent protein were constructed in which the circularly permuted Mapple protein [SEQ ID NO:3] was fused to a truncated PKCδ and in which the red fluorescence of the sensor increased upon binding to DAG. The complete amino acid sequence of the sensors are provided as SEQ ID NO:34, SEQ ID NO:35 and SEQ ID NO:36.

Example 2 Illustrates that a DAG Sensor can be Coupled with a Calcium Sensor in a Fusion Protein to Simultaneously Measure DAG and Calcium Signaling in Single Cells

To multiplex the expression of the DAG sensor with a Ca²⁺ sensor, we fused the coding regions of Green Upward DAG or Green Downward DAG to a cotranslational self-cleaving 2A [Szymczak et al.] peptide followed by R-GECO1 (FIG. 2B) to produce stoichiometrically balanced proportions of the two sensors. R-GECO1 is a red fluorescent Ca²⁺ sensor described in Zhao et al [17] based on a circularly permuted red fluorescent protein mApple [Shaner et al] with excitation and emission properties that are easily distinguished from the green fluorescent DAG sensors. In cells transiently expressing this dual sensor system, stimulation of the M1 receptor produced a fast rise in intracellular Ca2+, as detected by changes in the red fluorescence channel, and a much slower rise in DAG, as detected in the green fluorescence channel (FIG. 4). The Ca2+ returns to baseline in ˜20 seconds, while the DAG levels remain high for 200-300 seconds. This occurs for either the Downward or Upward DAG sensors paired with R-GECO1.

To test for the independence of the signals being detected by these sensors, we increased intracellular Ca2+ by applying ionomycin. This triggered a robust R-GECO1 response and no detectable change in the DAG sensor, which was subsequently activated by the addition of PDBu (FIG. 4C).

Example 3 Illustrates that DAG Sensors can be Co-Expressed with a Calcium Sensor to Simultaneously Measure DAG and Calcium Signaling in Single Cells

The Green Upward DAG2 and Downward DAG were co-expressed with the red fluorescent R-GECO1 to simultaneously measure Ca2+ and DAG signaling in living cells. The responses (mean pixel intensity) of individual cells upon stimulation of the M1 receptor by carbachol are plotted in FIG. 5A, the left axis is green fluorescence (arbitrary units) and the right axis represents red fluorescence. Both the onset of the Ca2+ response and the return to baseline was considerably quicker than the DAG response, which is consistent with previous measurements (24, 25).

Many compound libraries are carried by DMSO, a vehicle that can cause artifacts in live cell assays. To test the effects of DMSO on the DAG assay, DMSO of different concentrations was added to the culture, followed later by carbachol to evoke the maximal sensor response (FIG. 5B). At moderate final concentrations of 0.1 to 1%, the DMSO produced no effect, while at higher concentrations artifactual, DMSO triggered changes in fluorescence did occur. The effect of DMSO on the DAG sensors was negligible at final concentrations of 0.1 to 1%, but detectable at 2% or greater.

Example 4 Illustrates the Design and Construction of a PIP2 Sensor

Phospholipase C hydrolyses PIP2 to produce both DAG and IP3. To independently check the fidelity and kinetics of the DAG sensors, we created a red fluorescent PIP2 sensor by fusing the pleckstrin homology (PH) domain of PLCδ to two different components of the recently described dimerization-dependent red fluorescent proteins (26). Previous work has shown that the translocation of the PLCδ PH domain can be used to measure PIP2 turnover (27), and if the PH domains carry FRET pairs of fluorescent proteins a small change in FRET occurs when PLC is activated (28). To create a more robust sensor that does not involve FRET, and uses less of the visible spectrum, we fused the PH domain of PLCδ to each member of the ddRFP pair (26). One pair of constructs produced a particularly strong red fluorescent signal at the membrane that rapidly disappeared with M1 receptor activation (FIGS. 6A and B). Because this sensor relies on the fluorescence of one protein, rather than a FRET pair, it could be multiplexed with the green fluorescent DAG sensors for simultaneous measurement of both PIP2 and DAG in living cells. The amino acid sequences of this pair of constructs forming a novel PIP2 sensor are shown as SEQ ID NO:70 and SEQ ID NO:71, and the nucleotide sequences encoding them are shown as SEQ ID NO:72 and SEQ ID NO:73.

Example 5 Illustrates Multiplexing with DAG, PIP2, and Ca2+ Sensors

The red PIP2 sensor was coexpressed with the G-GECO1 Ca²⁺ sensor (17) and the M1 receptor. Stimulation of the M1 receptor by carbachol addition produced a rapid, simultaneous rise in Ca2+ and fall in PIP2 levels. (FIGS. 6A and B). Changes in Ca2+ can have profound effects on many cellular processes. To explore the relationship between intracellular Ca2+ levels and the signals produced by our DAG and PIP2 sensors, we first raised Ca2+ levels by adding the ionophore ionomycin, and then activated the DAG sensors with PdBU and the PIP2r sensors with M1 receptor activation. Raising intracellular Ca2+ had no apparent effect on the DAG and PIP2 levels (FIGS. 6C and D).

Example 6 Illustrates the Use of Multiplex PIP2 and DAG Sensors in Detection of PLC Activation

Stimulation of phospholipase C cleaves PIP2 and produces DAG. Co-expression of the Upward DAG2 or Downward DAG2 sensor with the PIP2r sensor provides a new view of both the substrate and product of phospholipase C (FIG. 7). To our knowledge, this is the first time that genetically encoded biosensors have been used to simultaneously measure substrate and product. M1 receptor activation produced a remarkable change in the intensity and distribution of both sensors. As expected, the PIP2r sensor rapidly leaves the membrane and the red fluorescence decreases while the Upward DAG sensor translocates to the membrane and the green fluorescence increases (FIG. 7A). The onset of the response of the Upward and Downward DAG2 sensors, as well as PIP2r, is kinetically quite similar. However the return to baseline is considerably slower for the Downward DAG2 and PIP2r sensors (FIG. 7B). Because this return to baseline varies depending upon our sampling rate, our interpretation is that the apparent return to baseline for Upward DAG2 is artificially accelerated by photobleaching, and similarly prolonged in the cases of Downward DAG2 and PIP2r.

Example 7 Illustrates the Use of Multiplex DAG and Calcium Sensors

It has been reported that ATP acting at the P2Y11 receptor produces inositol phosphate turnover and transient Ca2+ signaling consistent with Gq signaling, while UTP acting at the same receptor only triggers a Ca2+ response (29). To explore whether multiplex sensors could be used to detect this distinct signaling pattern, we expressed the human P2Y11 receptor with combinations of the Downward DAG2, or Upward DAG2, and R-GECO1 sensors. In HEK 293 cells, both ATP and UTP triggered a Ca2+ response that was identical in terms of kinetics and is consistent with receptor activation. (FIG. 8). The Upward and Downward DAG2 sensors, however, revealed that the ATP triggers signaling via the phospholipase C pathway, while the UTP is causing a Ca2+ transient in a very different way. This UTP effect could be seen in cells that expressed only the sensor, without the P2Y11 receptor, so these results are likely to be due to the action of UTP on a receptor intrinsic to this cell line, unlike what White and colleagues saw with a different cell line (29).

Example 8 Illustrates that the DAG Sensors Described Here are Compatible with Automated Drug Discovery

Protein-based, fluorescent biosensors have often worked at the microscope, under exacting experimental control, and failed to make an impact on the field of laboratory automation and screening. To test whether the fluorescent DAG sensors described here would be suitable for routine applications and automated screening, we co-expressed the M1 or P2Y11 receptors with the Downward DAG2 sensor in HEK293T cells plated on a 96 well, Corning Co-Star polystyrene plate. Media was replaced with PBS, and the fluorescence of each well before and after the addition of drug was measured using a standard plate reader. The change in fluorescence was measured for addition of vehicle alone as well as vehicle carrying carbachol or ATP. Using only the signal provided by Downward DAG2, we were able to observe a consistent, reproducible signal (Z′ values of 0.6 or greater) ((30) FIG. 9A).

Multiplex sensors offer the opportunity to improve an assay by making multiple, simultaneous, independent measurements. When both the green and red fluorescence measurements were captured from wells of cells expressing both the R-GECO1 Ca2+ sensor and the Downward DAG2 sensor, it was possible to plot the response to M1 receptor activation in terms of both sensors. Multiplexing the DAG sensors with R-GECO produces a two dimensional surface on which the negative control wells and positive carbachol responses are unambiguously separated (FIG. 9B). This reveals that there is a strong correlation between the amplitude of the two signals, and even more importantly, that the two independent signals can be used to increase the stringency of the assay, and separation between stimulated and unstimulated cells (FIG. 9B).

While various embodiments of the present invention have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. It is to be expressly understood, however, that such modifications and adaptations are within the scope of the present invention, as set forth in the following claims. The examples are provided for the purpose of illustration and are not intended to limit the scope of the present invention. Each publication, sequence or other reference disclosed below and elsewhere herein is incorporated herein by reference in its entirety, to the extent that there is no inconsistency with the present disclosure.

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TABLE 1 Candidate sensor structural information, including cpEGFP positions, truncation sites, and amino acid deletions in PKC delta. cpEGFP Truncation Sensor position site Deletion PcpG1 C280 PcpG2 I282 PcpG3 L286 PcpG4 A290 PcpG5 Q296 PcpG6 S302 PcpG7 E308 PcpG8 Y313 PcpG9 T320 PcpG10 E325 PcpG11 G332 PcpG12 I337 PcpG13 K343 PcpG14 N348 PcpG15 Y448 PcpG16 D217 PcpG17 N158 PcpG1 2B C280 L122 PcpG2 2B I282 L122 PcpG3 2B L286 L122 PcpG4 2B A290 L122 PcpG5 2B Q296 L122 PcpG6 2B S302 L122 PcpG7 2B E308 L122 PcpG8 2B Y313 L122 PcpG9 2B T320 L122 PcpG10 2B E325 L122 PcpG11 2B G332 L122 PcpG12 2B I337 L122 PcpG13 2B K343 L122 PcpG14 2B N348 L122 PcpG15 2B Y448 L122 PcpG16 2B D217 L122 PcpG17 2A N158 L91 PcpG17 2B N158 L122 PcpG17 2C N158 L106 PcpG17 2D N158 Q129 PcpG17 2A N158 K138 PcpG18 2B K157 L122 PcpG19 2B I156 L122 PcpG20 2B Y155 L122 PcpG21 2B H154 L122 PcpG22 2B I153 L122 PcpG23 2B K152 L122 Downward DAG PcpG24 2B H159 L122 PcpG25 2B E160 L122 PcpG26 2B F161 L122 PcpG27 2B I162 L122 PcpG28 2B A163 L122 PcpG29 2B T164 L122 PcpG30 2B E134 L122 PcpG1-2 C280 G281-I282 PcpG1-3 C280 G281-L286 PcpG1-4 C280 G281-A290 PcpG1-5 C280 G281-Q296 PcpG1-6 C280 G281-S302 PcpG1-7 C280 G281-E308 PcpG1-8 C280 G281-Y313 PcpG1-9 C280 G281-T320 PcpG1-10 C280 G281-E325 PcpG1-11 C280 G281-G332 PcpG1-12 C280 G281-I337 PcpG1-13 C280 G281-K343 PcpG1-14 C280 G281-N348

TABLE 2 DAG Sensing Domain

All sensors contain an N-terminal trucation of PKC delta that eliminates the C2 domain. Truncation at L122. All sensors contain E123-Q150 with   one exception noted, *PcpG30-21 sensor does not follow the general organization of the table. In this sensor, the insertion site of eGFP was  upstream of the insertion sites in the rest of the sensors after position E134. Amino acids D135-H154 are deleted in this sensor. deletion of PKC delta sensor domain residues indicated by - insertions indicated in bold italics 

What is claimed is:
 1. A nucleic acid molecule comprising an expression control sequence operably linked to a nucleotide sequence encoding a protein comprising a. a protein kinase C (PKC) protein comprising a DAG binding domain and a fusion region; and, b. a circularly permuted fluorescent protein, wherein the fusion region is located between the pseudo substrate domain and the DAG binding domain, or within the DAG binding domain; wherein the fluorescent protein is fused with the PKC protein at a fusion site present within the fusion region; and, wherein the fluorescence of the DAG sensor fusion protein changes upon binding to DAG.
 2. The nucleic acid molecule of claim 1, wherein the expression control sequence comprises one or more components selected from the group consisting of a promoter sequence, an enhancer sequence, a response element, a protein recognition site, a protein binding site, and a transcription start site.
 3. The nucleic acid molecule of claim 2, wherein the promoter is an inducible promoter, a constitutive promoter, or a cell-specific promoter.
 4. The nucleic acid molecule of claim 2, wherein the promoter is selected from the group consisting of a mammalian promoter, a polyoma virus promoter, a Simian Virus 40 (SV4) promoter, an adenovirus promoter, a retroviral promoter, a hepatitis B virus promoter, a Herpes virus promoter, and a cytomegalovirus (CMV) promoter.
 5. The nucleic acid molecule of claim 2, wherein the promoter is a beta-actin promoter, a EF1-alpha promoter, hybrids thereof, or combinations thereof.
 6. The nucleic acid molecule of claim 1, wherein the PKC protein is selected from the group consisting of PKC-δ (delta), PKC-ε (epsilon), PKC-θ (theta), PKC-η (eta), PKC-α (alpha), PKC-β1 (beta 1), PKC-β11 (beta 11), PKC-γ (gamma), and PKC-ξ (zeta).
 7. The nucleic acid molecule of claim 1, wherein the fusion region comprises an addition or deletion up to 10 amino acids.
 8. The nucleic acid molecule of claim 1, wherein the fusion region further comprises linker sequences.
 9. The nucleic acid molecule of claim 1, wherein the encoded protein comprises an amino acid sequence at least 90% identical to an amino acid sequence selected from the group consisting of SEQ ID NO:4, SEQ ID NO:5, SEQ ID NO:6, SEQ ID NO:7, SEQ ID NO:8, SEQ ID NO:9, SEQ ID NO:10, SEQ ID NO:11, SEQ ID NO:12, SEQ ID NO:13, SEQ ID NO:14, SEQ ID NO:15, SEQ ID NO:16, SEQ ID NO:17, SEQ ID NO:18, SEQ ID NO:19, SEQ ID NO:20, SEQ ID NO:21, SEQ ID NO:22, SEQ ID NO:23, SEQ ID NO:24, SEQ ID NO:25, SEQ ID NO:26, SEQ ID NO:27, SEQ ID NO:28, SEQ ID NO:29, SEQ ID NO:30, SEQ ID NO:31, SEQ ID NO:32, SEQ ID NO:33, SEQ ID NO:34, SEQ ID NO:35, and SEQ ID NO:36.
 10. The nucleic acid molecule of claim 1, comprising a nucleotide sequence selected from the group consisting of SEQ ID NO:37, SEQ ID NO:38, SEQ ID NO:39, SEQ ID NO:40, SEQ ID NO:41, SEQ ID NO:42, SEQ ID NO:43, SEQ ID NO:44, SEQ ID NO:45, SEQ ID NO:46, SEQ ID NO:47, SEQ ID NO:48, SEQ ID NO:49, SEQ ID NO:50, SEQ ID NO:51, SEQ ID NO:52, SEQ ID NO:53, SEQ ID NO:54, SEQ ID NO:55, SEQ ID NO:56, SEQ ID NO:57, SEQ ID NO:58, SEQ ID NO:59, SEQ ID NO:60, SEQ ID NO:61, SEQ ID NO:62, SEQ ID NO:63, SEQ ID NO:64, SEQ ID NO:65, and SEQ ID NO:66.
 11. A kit comprising the nucleic acid molecule of claim
 1. 12. A viral vector comprising the nucleic acid molecule of claim
 1. 13. A cell comprising the nucleic acid molecule of claim
 1. 14. The cell of claim 13, further comprising one or more additional nucleic acid molecules that encode one or more additional fluorescent sensor proteins that specifically detect an analyte other than DAG.
 15. A method of detecting diacylglycerol in a cell, comprising: a. exposing a cell comprising the nucleic acid molecule of claim 1 to light that excites the circularly permuted fluorescent protein; and, b. measuring the amount of fluorescent light produced by the cell at a wavelength corresponding to the emission wavelength of the fluorescent protein, thereby detecting diacylglycerol in the cell.
 16. A method of detecting activation of the phospholipase C (PLC) pathway in a cell, comprising: a. exposing a cell comprising the nucleic acid molecule of claim 1 to light that excites the circularly permuted fluorescent protein; and, b. determining the level of diacylglycerol in the cell by measuring the amount of fluorescent light produced by the cell at a wavelength corresponding to the emission wavelength of the fluorescent protein; wherein a level of diacylglycerol greater than a basal level indicates activation of the phospholipase C pathway.
 17. A method of identifying a compound that modulates the level of diacylglycerol in a cell, comprising: contacting a test compound with a cell comprising the nucleic acid molecule of claim 1; exposing the cell to light that excites the circularly permuted fluorescent protein; and, measuring the amount of fluorescent light produced by the cell at a wavelength corresponding to the emission wavelength of the fluorescent protein; wherein a change in the amount of fluorescent light produced by the cell indicates the test compound modulates the level of diacylglycerol in the cell.
 18. A method of identifying a compound capable of modulating the activity of the phospholipase C (PLC) pathway in a cell, comprising: a. contacting a test compound with a cell comprising the nucleic acid molecule of claim 1; b. exposing the cell to light that excites the circularly permuted fluorescent protein; and, c. measuring the amount of fluorescent light produced by the cell at a wavelength corresponding to the emission wavelength of the fluorescent protein; wherein a change in the amount of fluorescent light produced by the cell indicates the compound modulates the PLC pathway. 